Chapter 6. The IObjectWrapper and Type conversion

The concepts encapsulated by the IObjectWrapper interface are fundamental to the workings of the core Spring.NET libraries The typical application developer most probably will not ever have the need to use the IObjectWrapper directly... because this is reference documentation however, we felt that some explanation of this core interface might be right. The IObjectWrapper is explained in this chapter since if you were going to use it at all, you would probably do that when trying to bind data to objects, which, nicely enough, is precisely the area that the IObjectWrapper addresses.

One quite important concept of the Spring.Objects namespace is encapsulated in the definition IObjectWrapper interface and its corresponding implementation, the ObjectWrapper class. The functionality offered by the IObjectWrapper includes methods to set and get property values (either individually or in bulk), get property descriptors (instances of the System.Reflection.PropertyInfo class), and to query the readability and writability of properties. The IObjectWrapper also offers support for nested properties, enabling the setting of properties on subproperties to an unlimited depth. The IObjectWrapper usually isn't used by application code directly, but by framework classes such as the various IObjectFactory implementations.

The way the IObjectWrapper works is partly indicated by its name: it wraps an object to perform actions on a wrapped object instance... such actions would include the setting and getting of properties exposed on the wrapped object.

Note: the concepts explained in this section are not important to you if you're not planning to work with the IObjectWrapper directly.

[C#]
public class Company
  {
    private string name;
    private Employee managingDirector;

    public string Name
    {
      get { return this.name; }
      set { this.name = value; }
    }
    
    public Employee ManagingDirector
    {
      get { return this.managingDirector; }
      set { this.managingDirector = value; }
    }
}

[C#]
public class Employee
{
    private string name;
    private float salary;
    
    public string Name
    {
      get { return this.name; }
      set { this.name = value; }
    }

    public float Salary
    {
      get { return salary; }
      set { this.salary = value; }
    }
}

[C#]
Company c = new Company();
IObjectWrapper owComp = new ObjectWrapper(c);
// setting the company name...
owComp.SetPropertyValue("name", "Salina Inc.");
// can also be done like this...
PropertyValue v = new PropertyValue("name", "Salina Inc.");
owComp.SetPropertyValue(v);

// ok, let's create the director and bind it to the company...
Employee don = new Employee();
IObjectWrapper owDon = new ObjectWrapper(don);
owDon.SetPropertyValue("name", "Don Fabrizio");
owComp.SetPropertyValue("managingDirector", don);

// retrieving the salary of the ManagingDirector through the company
float salary = (float)owComp.GetPropertyValue("managingDirector.salary");

[C#]
// ok, let's create the director and bind it to the company...
Employee don = new Employee();
IObjectWrapper owDon = new ObjectWrapper(don);
owDon.SetPropertyValue("naMe", "Don Fabrizio");
owDon.GetPropertyValue("nAmE"); // gets "Don Fabrizio"

IObjectWrapper owComp = new ObjectWrapper(new Company());
owComp.SetPropertyValue("ManaGINGdirecToR", don);
owComp.SetPropertyValue("mANaGiNgdirector.salARY", 80000);
Console.WriteLine(don.Salary); // puts 80000

If you associate a TypeConverter with the definition of a custom Type using the standard .NET mechanism (see the example code below), Spring.NET will use the associated TypeConverter to do the conversion.

[C#]
[TypeConverter (typeof (FooTypeConverter))]
public class Foo
{
}

The TypeConverter class from the System.ComponentModel namespace of the .NET BCL is used extensively by the various classes in the Spring.Core library, as said class “... provides a unified way of converting types of values to other types, as well as for accessing standard values and subproperties.” ^[4]

For example, a date can be represented in a human readable format (such as 30th August 1984), while we're still able to convert the human readable form to the original date format or (even better) to an instance of the System.DateTime class. This behavior can be achieved by using the standard .NET idiom of decorating a class with the TypeConverterAttribute. Spring.NET also offers another means of associating a TypeConverters with a class. You might want to do this to achieve a conversion that is not possible using standard idiom... for example, the Spring.Core library contains a custom TypeConverter that converts comma-delimited strings to String array instances. Registering custom converters on an IObjectWrapper instance gives the wrapper the knowledge of how to convert properties to the desired Type.

An example of where property conversion is used in Spring.NET is the setting of properties on objects, accomplished using the aforementioned TypeConverters. When mentioning System.String as the value of a property of some object (declared in an XML file for instance), Spring.NET will (if the type of the associated property is System.Type) use the RuntimeTypeConverter class to try to resolve the property value to a Type object. The example below demonstrates this automatic conversion of the Example.Xml.SAXParser (a string) into the corresponding Type instance for use in this factory-style class.

<objects xmlns="http://www.springframework.net">
<object id="parserFactory" type="Example.XmlParserFactory, ExamplesLibrary"
destroy-method="Close">
  <property name="ParserClass" value="Example.Xml.SAXParser, ExamplesLibrary"/>
</object>
</objects>

[C#]
public class XmlParserFactory
{
	private Type parserClass;
	
	public Type ParserClass
	{
	  get { return this.parserClass; }
	  set { this.parserClass = value; }
	}
	
	public XmlParser GetParser ()
	{
	  return Activator.CreateInstance (ParserClass);
	}
}

<object id="rod" type="Spring.Objects.TestObject, Spring.Core.Tests">
  <property name="name" value="Rod"/>
  <property name="FileMode" value="Create"/>
</object>

Spring.NET has a number of built-in TypeConverters to make life easy. Each of those is listed below and they are all located in the Spring.Objects.TypeConverters namespace of the Spring.Core library.

Spring.NET uses the standard .NET mechanisms for the resolution of System.Types, including, but not limited to checking any configuration files associated with your application, checking the Global Assembly Cache (GAC), and assembly probing.